1. Field of the Invention
The present invention relates to the use of an adaptive array in radio communications and in particular to improve the performance of communications and broadcast systems that incorporate multiple signals.
2. Description of the Prior Art
To obtain high quality reception, communications receivers require a signal that is uncorrupted by interference, such as multipath, adjacent channel or co-channel. One source of interference that can severely degrade reception is multipath. Multipath occurs when the transmitted signal arrives at the receiver's antenna over different propagation paths resulting in different time delays and phase shifts. The multiple paths are generally due to reflections of the transmitted signal from hills, buildings, etc. and can also be the result of atmospheric phenomena. Multipath can cause distortion in the amplitude, phase and frequency of the received signal, which can result in deep signal strength fades, frequency selective fades, intersymbol interference, noise, etc.
The same transmitting antenna can be used for the transmission of two radio communications signals at the same time in the same frequency channel. At the receiving end, the two radio communications signals can be received by the same receiving antenna at the same time before being coupled to their individual receivers. Both radio signals can suffer from interference (multipath and/or other shared interference) problems and require that the interference be reduced to improve reception.
Two approaches, well known in the art for reducing the effects of multipath and other interference, are the adaptive array and adaptive filter (see for example Widrow, B. & others, “Adaptive Antenna Systems”, Proceedings of the IEEE, Vol. 55, No. 12, December 1967, pp. 2143-2159; Treichler, John R., Johnson, C. Richard Jr., and Larimore, Michael G., Theory and Design of Adaptive Filters, John Wiley & Sons, New York, 1987; Monzingo, Robert A. and Miller, Thomas W., Introduction to Adaptive Arrays, John Wiley & Sons, New York, 1980; Compton, R. T. Jr., Adaptive Antenna Concepts and Performance, Prentice Hall, Englewood Cliffs, N.J., 1988; U.S. Pat. Nos. 4,736,460 and 4,752,969 by Kenneth Rilling, and others). When multiple signals are present, the use of an adaptive array or adaptive filter for each signal can be expensive, complicated and occupy too much space. For some signals, a dedicated adaptive array or adaptive filter may not provide enough performance improvement if the signal structure or available information does not lend itself to the use of an adaptive array or adaptive filter.